1. Field of Invention
This invention relates to space thermostats and, in particular, to space thermostats having anticipator circuits for the heating and/or cooling cycles.
2. Brief Statement of the Prior Art
Space thermostats typically have a bimetallic switch operator such as a bimetallic helical coil to open and close the thermostat in response to ambient temperature changes. The thermostats have, for many years, included anticipator circuits in which a resistive heating element is included within the thermostat housing in circuit with the thermostat contacts to release heat to the thermostat housing, raising the bimetallic switch actuator slightly above the ambient temperature and thereby anticipating the temperature rise which occurs after opening of the thermostat contacts and discontinuance of the heating cycle from inertia of the heating system.
The most commonly employed heating resistor is a small diameter wire conductor with a slidable contact moveable along its length to provide fixed adjustability in the value of the heating resistor.
A difficulty with the conventional anticipator construction is that the heat transfer between the resistance heater and the bimetallic switch actuator has, heretofore, depended on juxtapositioning these elements. The inefficient heat transfer between these members requires a substantial release of heat to the thermostat housing for adequate anticipation and the amount of heat so released often can not be dissipated during the portion of the heating cycle when the anticipator circuit is open. The resultant performance has been referred to as "droop". The slide wire variable resistor heating element also can burn out if the slide wire contact is incorrectly positioned for the particular application. Finally, the slide wire resistance heater provides a linear or constant value of resistivity along its length and the non-linear heat release of the anticipator results in a non-linear or geometrical scale for the slide contact, greatly decreasing the useful application range of the instrument.